It is known that for extreme ultraviolet (“EUV”) light, e.g., at wavelengths of around 20 nm or less, sometimes also referred to a soft-xrays, e.g., at 13.5 nm, reflective optical elements will be needed, e.g., for collecting and focusing the EUV light generated from a plasma created from a source material. At the wavelengths involved, either grazing angle of incidence or multi-layer mirror (“MLM”) so called normal angle of incidence reflectors will be necessary for the collection and focusing of the light emitted from the plasma, whether an electric discharge produced plasma (“DPP”) produced by an electrical discharge between a pair of electrodes or a laser produced plasma (“LPP”) produced by a focused laser beam irradiating a target material to produce the plasma.
In the process of the creation of the plasma for the emission of EUV light several harsh results of the plasma creation are released into the environment in the EUV light source generation chamber around the plasma that are potentially very damaging to materials comprising, e.g., the collector/director optical element, e.g., heat, high energy ions and scattered debris from the plasma formation, e.g., atoms or clumps of source material not ionized in the plasma formation process. The heat, high energy ions and/or source material may be damaging to the optical elements in a number of ways, including simply heating them, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the mechanics of MLM operation to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. The source materials may be particularly reactive, e.g., with a material making up at least one layer of the MLM, e.g., lithium and silicon, so that steps may need to be taken to reduce the potential effects of the reactivity, especially at elevated temperatures, and/or keep the materials separated, e.g., either by capping layers or by intermediate diffusion barrier layers or both. Temperature stability, ion-implantation and diffusion problems may need to be addressed even with less reactive source materials, e.g., tin, or indium or xenon.
In addition, the nature of debris management for the EUV light source chamber may result in increasing the harshness of the environment in which the MLM stack and its protective overcoating (capping) layer(s) need to operate and protect the underlying binary multilayer stack. This needs to be done without detracting significantly from overall reflectivity at the same time. Technique employed may be, e.g., heating the reflector to elevated temperatures of, e.g., over 500° C., e.g., to evaporate debris from the reflector surfaces and/or utilizing an etchant, e.g., a halogen etchant to etch debris from the reflector surfaces and/or creating a shielding plasma in the vicinity of the reflector surfaces, as discussed further in the above referenced co-pending patent application.
Applicants propose a variety of MLM arrangements and materials useful in optimizing the reflectivity of the optical elements to incident EUV light and the lifetime of the optical elements in the harsh environment, where, by way of example some 16 to 48 thousand plasma formations may occur per second of operation of the plasma generated EUV light source in close proximity to the collector/director and other optical elements in a light source chamber in which the optical elements must remain for months if not a year or more at a time, due to the difficulties engendered by breaking the seal of the light source chamber to replace optical elements and due to the expense of replacement of such optical elements.
Some have discussed lithium compatibilities with other materials and lithium diffusion, but not in the context of MLM and particularly not in the context of providing a suitable collector/director for an EUV light source with a reactive plasma source materials and specifically not in the context of a lithium plasma source material. M. Eckhardt, et al. “Influence of doping on the bulk diffusion of Li into Si(100)”, Surf. Sci. 319, 219-223 (1994) discusses the influence of doping on bulk diffusion of Li into a Si crystal. The article describes the influence of doping on the bulk diffusion of Li into Si(100). They state that for an n-type doped Si(100) surface there is no lithium diffusion into the bulk at temperatures below 1000 K (=730° C.). Applicants propose to apply this principle to the provision of suitable EUV optical reflecting elements where the source material for the plasma is a reactive element, e.g., lithium.
Yttria has been used by the fusion community to protect the first reactor wall from the hot lithium by means of a coating. Also work on Mo/Y multilayers has been done by Livermore Laboratories National Laboratories (“LLNL”) for reflection in the wavelength range of 7-12 nm. However applicants are not aware of use of yttrium for collector/director or other optics in a plasma generated EUV light source for protection of the optics, e.g., from a reactive source material, e.g., lithium.
Also see Compatibility of insulating ceramic materials with liquid breeders, Mitsuyama et al., Fusion Eng. Des. 39-40, 811(1998), Pint et al., “High temperature compatibility issues for fusion reactor structural materials”, Fusion Sci. Technol. 44, 433-440 (2003); Sarafat, et al., “Coolant structural materials compatibility,” Report, Apex meeting, Mar. 24, 2000; Kloidt et al, Appl. Phys. Lett. 58 (23), 2601-2603 (1991)
Others have discussed MLM materials and properties, but not in the context of plasma generated EUV light sources and also not reactive source materials, and particularly not in the context of the use of lithium as a plasma source material. Several patents and articles have discussed MLM materials and capping layers, but not in the context of the requirements for a plasma generated EUV collector/director and other EUV source chamber optics, e.g., temperature stability requirements at relatively elevated temperatures, and also not in the context of a reactive EUV plasma material and particularly lithium. Mo/Y MLMs without barrier layers have been shown to be thermally stable to 250° C. by Bajt et al., LLNL group, e.g., in Bajt, et al., “Mo:Y multilayer mirror technology utilized to image the near-field output of a Ni-like sn laser at 11.9 nm”, Optics Letters, Vol. 28, No. 22 (Nov. 15, 2003 p. 2249, and Kjornrattanawanich, “Reflectance, optical properties and stability of molybdenum/strontium and molybdenum/yttrium multilayer mirrors, Ph. D. Dissertation (University of California Davis, Report UCRL-LR-150541 (2002). If yttrium layers have just the right very small amount of oxygen or if they are essentially oxygen-free then Mo/Y multilayer may be stable also at temperatures above 250° C., as indicated by the referenced Kjornrattanawanich Dissertation, where it was observed that there is a higher contrast for Mo/Y multilayers in cross-sectional transmission-electron microscope pictures after heating (annealing) of the Mo/Y mirrors. U.S. Pat. No. 6,724,462, issued to Singh, et al. on Apr. 20, 2004, entitled CAPPING LAYER FOR EUV OPTICAL ELEMENTS, discusses EUV reflectors for lithography tool environments not subject to the rigors of the environment within a plasma produced EUV light source that must be accounted for in selecting appropriate materials for the reflectors, including, e.g., the choice between grazing angle or incidence reflecting layers and multilayer mirrors for more normal angle of incidence, the shape and proximity of reflector surfaces to the plasma, the plasma source material, debris mitigation steps taken, e.g., elevated temperatures for debris evaporation, halogen debris etching, debris diffusion, etc. Rather the materials selected by the '462 patent and others are based almost exclusive on maximizing reflectivity in a relatively sterile and pristine environment of a lithography tool utilizing EUV light for photoresist exposure, where, e.g., the capping layer is selected to be “relatively inert” to the surrounding environment, e.g., exposure to air. To similar effect is U.S. Pat. No. 6,656,575, issued to Bijkerk, et al. on Dec. 2, 2003, MULTILAYER SYSTEM WITH PROTECTING LAYER SYSTEM AND PRODUCTION METHOD, relating also to a lithography tool environment for EUV reflectors. U.S. Pat. No. 6,449,086, issued to Singh on Sep. 10, 2002, entitled MULTILAYER EXTREME ULTRAVIOLET MIRRORS WITH ENHANCED REFLECTIVITY is to the same effect relating to intermediate layer materials and a capping layer of “relatively inert material.” U.S. Pat. No. 6,228,512, issued to Bajt, et al. on May 8, 2001, entitled MORU/BE MULTILAYERS FOR EXTREME ULTRAVIOLET APPLICATIONS, relating to MoRu/Be MLM binary layers and roughness reducing and intermixing intermediate layers and oxide capping layers for systems potentially exposed to water vapor. U.S. Pat. No. 6,780,496, issued to Bajt, et al. on Aug. 24, 2004, entitled, OPTIMIZED CAPPING LAYERS FOR EUV MULTILAYERS including a binary capping layer with Ru and an undercoating to prevent Ru diffusion into the underlying binary layers and Ru selected for resistance to oxidation in, e.g., a lithography tool environment.
Takenaka et al., “Heat resistance of Mo/Si, MoSi2/Si and Mo5Si3/Si multilayer soft x-ray mirrors”, J. Appl. Phys. 78, 5227 (1995) discusses the combination Mo5Si3/Si, but not the combination Si—MoSi2—Mo5Si3—MoSi2 proposed by applicants.